It turns out that Gmail already has a built-in group for Google Buzz contacts. The group doesn't have a name and it's not displayed in Gmail's new contact manager, but you can find it in the old version of Gmail: it's the only one without a name.
Since the group doesn't have a name and it's hidden in the interface, you can't use it to send email messages or to post private Buzz messages, but you can select all the contacts and add them to another group.
There's also a hidden group for Google Latitude friends, which includes the people that can see your location in Google Latitude.
Another group lists all your Gmail Chat/Google Talk friends. Some of these people were automatically added by Google if you didn't disable "Automatically allow people I communicate with often to chat with me and see when I'm online" in the settings.
For those who miss the "all contacts" group in the new contact manager, here's the built-in group that includes both the people you've manually added ("my contacts") and the people automatically added by Google ("other contacts").